Transcript
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Recap: Connection setup
•Three-way handshake:• Client sends SYN(x)• Server replies with SYN(y)ACK(x+1)• Client replies with ACK(y+1)• SYNs are retransmitted if lost
•Sequence and ack numbers carried on further segments
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2
3
Active party(client)
Passive party(server)
SYN (SEQ=x)
SYN (SEQ=y, ACK=x+1)
(SEQ=x+1, ACK=y+1)Time
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Three-Way Handshake
•Suppose delayed, duplicate copies of the SYN and ACK arrive at the server!• Improbable, but anyhow …
Active party(client)
Passive party(server)
SYN (SEQ=x)
(SEQ=x+1,ACK=z+1)
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Three-Way Handshake
•Suppose delayed, duplicate copies of the SYN and ACK arrive at the server!• Improbable, but anyhow …
•Connection will be cleanly rejected on both sides J
Active party(client)
Passive party(server)
SYN (SEQ=x)
SYN (SEQ=y, ACK=x+1)
(SEQ=x+1,ACK=z+1)
XXREJECT
REJECT
Connection Release
•Orderly release by both parties when done• Delivers all pending data and “hangs up”• Cleans up state in sender and receiver
•Key problem is to provide reliability while releasing• TCP uses a “symmetric” close in which both sides
shutdown independently
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TCP Connection Release
•Two steps:• Active sends FIN(x), passive ACKs• Passive sends FIN(y), active ACKs• FINs are retransmitted if lost
•Each FIN/ACK closes one direction of data transfer
Active party Passive party
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TCP Connection Release (2)
•Two steps:• Active sends FIN(x), passive ACKs• Passive sends FIN(y), active ACKs• FINs are retransmitted if lost
•Each FIN/ACK closes one direction of data transfer
Active party Passive party
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2
FIN (SEQ=x)
(SEQ=y, ACK=x+1)
(SEQ=x+1, ACK=y+1)
FIN (SEQ=y, ACK=x+1)
TCP Connection State Machine
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Both parties run instances of this state
machine
TCP Release (3)
•Again, with states …
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1
2
CLOSED
FIN (SEQ=x)
(SEQ=y, ACK=x+1)
(SEQ=x+1, ACK=y+1)
FIN (SEQ=y, ACK=x+1)
Active party Passive party
FIN_WAIT_1
CLOSE_WAIT
LAST_ACKFIN_WAIT_2
TIME_WAIT
CLOSED
ESTABLISHED
(timeout)
ESTABLISHED
TIME_WAIT State
•Wait a long time after sending all segments and before completing the close• Two times the maximum segment lifetime of 60 seconds
•Why?
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TIME_WAIT State
•Wait a long time after sending all segments and before completing the close• Two times the maximum segment lifetime of 60 seconds
•Why?• ACK might have been lost, in which case FIN will be resent
for an orderly close• Could otherwise interfere with a subsequent connection
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ARQ: Automatic repeat query
•ARQ with one message at a time is Stop-and-Wait
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Frame 0
ACK 0Timeout Time
Sender Receiver
Frame 1
ACK 1
Limitation of Stop-and-Wait
• It allows only a single message to be outstanding from the sender:• Fine for LAN (only one frame fits in network anyhow)• Not efficient for network paths with longer delays
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Limitation of Stop-and-Wait (2)
•Example: B=1 Mbps, D = 50 ms• RTT (Round Trip Time) = 2D = 100 ms• How many packets/sec? • 10
• Usage efficiency if packets are 10kb?• (10,000 x 10) / (1 x 106) = 10%
•What is the efficiency if B=10 Mbps?• 1%
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Sliding Window
•Generalization of stop-and-wait• Allows W packets to be outstanding• Can send W packets per RTT (=2D)
• Pipelining improves performance • Need W=2BD to fill network path
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Sliding Window (2)
What W will use the network capacity with 10kb packets?
• Ex: B=1 Mbps, D = 50 ms• 2BD = 2 x 106 x 50/1000 = 100 Kb• W = 100 kb/10 = 10 packets
• Ex: What if B=10 Mbps?• W = 100 packets
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Sliding Window Protocol
•Many variations, depending on how buffers, acknowledgements, and retransmissions are handled
•Go-Back-N• Simplest version, can be inefficient
•Selective Repeat•More complex, better performance
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Sender Sliding Window
•Sender buffers up to W segments until they are acknowledged• LFS=LAST FRAME SENT, LAR=LAST ACK REC’D• Sends while LFS – LAR ≤ W
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.. 5 6 7 .. 2 3 4 5 2 3 ..
LAR LFS
W=5
Acked Unacked 3 ..Unavailable
Available
seq. number
SlidingWindow
Sender Sliding Window (2)
•Transport accepts another segment of data from the Application ...• Transport sends it (LFS–LAR à 5)
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.. 5 6 7 .. 2 3 4 5 2 3 ..
LAR
W=5
Acked Unacked 3 ..Unavailable
Sent
seq. number
SlidingWindow
LFS
Sender Sliding Window (3)
•Next higher ACK arrives from peer…•Window advances, buffer is freed • LFS–LAR à 4 (can send one more)
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.. 5 6 7 .. 2 3 4 5 2 3 ..
LAR
W=5
Acked Unacked 3 ..Unavailable
Available
seq. number
SlidingWindow
LFS
Receiver Sliding Window – Go-Back-N
•Receiver keeps only a single packet buffer for the next segment• State variable, LAS = LAST ACK SENT
•On receive:• If seq. number is LAS+1, accept and pass it to app, update
LAS, send ACK• Otherwise discard (as out of order)
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Receiver Sliding Window – Selective Repeat
• Receiver passes data to app in order, and buffers out-of-order segments to reduce retransmissions
• ACK conveys highest in-order segment, plus hints about out-of-order segments• Ex: I got everything up to 42 (LAS), and got 44, 45
• TCP uses a selective repeat design; we’ll see the details later
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Receiver Sliding Window – Selective Repeat (2)
•Buffers W segments, keeps state variable LAS = LASTACK SENT
•On receive:• Buffer segments [LAS+1, LAS+W] • Send app in-order segments from LAS+1, and update LAS• Send ACK for LAS regardless
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5
Sender Sliding Window – Selective Repeat
•Keep normal sliding window• If out-of-order ACK arrives• Send LAR+1 again!
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.. 5 6 7 .. 2 4 5 3 ..
LAR
W=5
Acked Unacked 3 ..Unavailable
Ack Arrives Out of Order!
seq. number
SlidingWindow
LFS
..
5
Sender Sliding Window – Selective Repeat (2)
•Keep normal sliding window• If in-order ACK arrives•Move window and LAR, send more messages
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.. 5 6 7 .. 4 5 3 ..
LAR
W=5
Acked Unacked 3 ..
In-order ack arrives…
seq. number
SlidingWindow
LFS
....Now Available
Sliding Window – Retransmissions
•Go-Back-N uses a single timer to detect losses• On timeout, resends buffered packets starting at LAR+1
•Selective Repeat uses a timer per unacked segment to detect losses• On timeout for segment, resend it• Hope to resend fewer segments
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Sequence Numbers
Need more than 0/1 for Stop-and-Wait … but how many?• For Selective Repeat: 2W seq numbers• W for packets, plus W for earlier acks
• For Go-Back-N: W+1 sequence numbers
Typically implement seq. number with an N-bit counter that wraps around at 2N—1 • E.g., N=8: …, 253, 254, 255, 0, 1, 2, 3, …
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Sequence Time Plot
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Time
Seq.
Num
ber
Acks(at Receiver)
Delay (=RTT/2)
Transmissions(at Sender)
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